Ramez Naam, author of Nexusand Crux, has speculated a lot about humans being merged with machines — but he's actually quite skeptical about how quickly this will happen in reality. He's also not sure the Singularity is coming in his lifetime.

The safety bar for doing a surgery to implant something in the human body (let along the human brain) is extremely high.

Indeed, brain surgery itself is biggest barrier to progress here.
We're going to need new, less invasive ways to interface brains and
electronics if we ever want this to take off. In Nexus I
proposed doing this by self-assembling nano-structures, each component
of which is small enough to cross the blood-brain barrier. It's sort of a
barely-plausible hand wave. Real neuroscientists, however, have
somewhat similar ideas. Rudolfo Llinas, who was the Editor in Chief of
the journal Neuroscience for 20 years, has proposed inserting somewhere between tens of thousands and millions of nanowires into the brain by sliding them into an artery somewhere else in your body. This approach needs no brain surgery.

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If we could upgrade our brains via implants, that would give us a huge advantage in the race against the machines — but meanwhile, we are already using our technology as a "cognitive prosthesis," says Naam.